Method for encoding a current block of a first image component relative to a reference block of at least one second image component, encoding device and corresponding computer program

ABSTRACT

A method for coding a current block of a first image component with respect to a reference block of at least one second image component. The first and second image components are representative of the same scene, and the reference block being previously subjected to coding by partitioning, and then to decoding. Partitioning of the reference block is performed a plurality of times until a determined level of depth of partitioning is obtained. The coding method includes, as a function of the type of the first and second image components: partitioning the current block a plurality of times until a level of depth of partitioning dependent on the level of partitioning of the reference block is obtained; or partitioning the current block on the basis of a level of depth of partitioning previously initialized by a level of partitioning depending on the level of partitioning for the reference block.

CROSS-REFERENCE TO RELATES APPLICATIONS

This application is a Section 371 National Stage application ofInternational Application No. PCT/FR2013/051474, filed Jun. 25, 2013,which is incorporated by reference in its entirety and published as WO2014/001703 on Jan. 3, 2014, not in English.

FIELD OF THE INVENTION

The present invention pertains generally to the field of imageprocessing, and more precisely to the coding of digital images and ofsequences of digital images.

The invention can thus in particular be applied to the video codingimplemented in current video coders in future (ITU-T/ISO MPEG HEVC) andtheir extensions.

BACKGROUND OF THE INVENTION

The HEVC standard currently being drafted and described in the document“B. Bross, W.-J. Han, J.-R. Ohm, G. J. Sullivan, and T. Wiegand, “Highefficiency video coding (HEVC) text specification draft 6,” documentJCTVC-H1003 of JCT-VC, San Jose Calif., USA, February 2012” is similarto the previous H.264 standard, in the sense that it uses blockpartitioning of the video sequence. The HEVC standard is, however,distinguished from the H.264 standard by the fact that the partitioningimplemented complies with a tree-like structure called “quadtree”. Forthis purpose, as shown in FIG. 1A, a current image I_(N) is partitioneda first time into a plurality of square blocks CTB₁, CTB₂, . . . ,CTB_(i), . . . , CTB_(T) of size 64×64 pixels (1≦i≦L). For a given blockCTB_(i), this block is considered to constitute the root of a codingtree in which:

-   -   a first level of leaves under the root corresponds to a first        level of partitioning depth for the block CTB_(i) for which the        block CTB_(i) has been partitioned a first time into a plurality        of coding blocks,    -   a second level of leaves under the first level of leaves        corresponds to a second level of partitioning depth for the        block CTB_(i) for which the block CTB_(i) partitioned a first        time is partitioned a second time into a plurality of coding        blocks, . . . .    -   . . . a kth level of leaves under the k−1th level of leaves        which corresponds to a kth level of partitioning depth for the        block CTB_(i) for which the block CTB_(i) partitioned k−1 times        is partitioned one last time into a plurality of coding blocks.

In an HEVC compatible coder, the iteration of the partitioning of theblock CTB_(i) is performed as far as a predetermined level ofpartitioning depth.

On completion of the aforementioned successive partitionings of theblock CTB_(i), as shown in FIG. 1A, the latter is partitioned in the endinto a plurality of coding blocks denoted CB₁, CB₂, . . . , CB_(j), . .. , CB_(M) with 1≦j≦M.

The size of said coding blocks can be chosen in an adaptive manner withthe aid of partitioning of blocks complying with a tree of “quadtree”type, in which the leaves of said tree respectively represent the codingblocks CB₁, CB₂, . . . , CB_(j), . . . , CB_(M) obtained at variouslevels of partitioning depth.

With reference to FIG. 1A, for a given block CB_(j), this block isconsidered to constitute the root of a prediction and transformationtree for said block, for example of discrete cosine transform (DCT)type. The prediction tree for a given block CB_(j) is representative ofthe way in which the block CB_(j) is partitioned into a plurality ofblocks PB₁, PB₂, . . . , PB_(t), . . . , PB_(P), (1≦t≦P), which arecalled prediction blocks. For a considered prediction block PB_(t),prediction parameters, such as for example the mode of coding, themotion vectors, etc., are specified in a prediction unit.

There are various modes of partitioning for a considered coding blockCB_(j). FIG. 1A shows for example the various modes of partitioning ofthe considered coding block CB_(j), in the case of an INTER predictionfor the latter. There are four of said modes of partitioning:

-   -   the PART_2N×2N mode corresponds to the absence of partitioning        of the considered coding block CB_(j), which thus corresponds to        a single prediction block PB₁,    -   the PART_2N×N mode corresponds to a horizontal partitioning of        the considered coding block CB_(j) into two rectangular        prediction blocks PB₁ and PB₂,    -   the PART_N×2N mode corresponds to a vertical partitioning of the        considered coding block CB_(j) into two rectangular prediction        blocks PB₁ and PB₂,    -   the PART_N×N mode corresponds to a partitioning of the        considered coding block CB_(j) into four square prediction        blocks PB₁, PB₂, PB₃, PB₄ which all have the same size.

After predictive coding of the considered coding block CB_(j), thelatter can be partitioned again into a plurality of smaller blocks TB₁,TB₂, . . . , TB_(v), . . . , TB_(Q) (1≦v≦Q), which are called transformblocks. Such partitioning complies with a tree of “quadtree” type,called a “residual quadtree”, in which the leaves of said treerespectively represent the coding blocks TB₁, TB₂, . . . , TB_(v), . . ., TB_(Q) obtained at various levels of partitioning depth.

FIG. 1A shows an exemplary partitioning of the coding block CB_(j) whichhas been predicted with the aid of the PART_N×N partitioning. In theexample shown, the blocks PB₂ and PB₃ of the coding block CB_(j) are forexample each partitioned into four smaller square blocks all of the samesize, TB₁, TB₂, TB₃, TB₄ and TB₅, TB₆, TB₇, TB₈, respectively. Suchpartitioning is represented with dashed lines in FIG. 1A.

FIG. 1B shows an exemplary partitioning of a considered block CTB_(i)which has been obtained after predictive coding and transform coding ofsaid block, as well as the corresponding partitioning tree. In theexample shown:

-   -   the block CTB_(i), considered to be the root of the coding tree,        is represented with a heavy continuous line,    -   the coding blocks CB₁ to CB₁₆, which constitute on the one hand        the leaves of the coding tree and on the other hand the roots of        the “residual quadtree” tree, are represented with fine        continuous lines,    -   the transform blocks TB₁ to TB₁₆, which constitute the leaves of        the “residual quadtree” tree, are represented with dashed lines.

In the tree-like structure constituted in this manner, there is:

-   -   a first level of partitioning depth NP1 which contains solely        coding blocks, such as the blocks CB₁ to CB₄,    -   a second level of partitioning depth NP2 which contains:        -   coding blocks, such as the blocks CB₅ to CB₈ obtained on            completion of the partitioning of the block CB₁, as well as            the blocks CB₉ to CB₁₂ obtained on completion of the            partitioning of the block CB₄,        -   transform blocks, such as the blocks TB₁ to TB₄ obtained on            completion of the partitioning of the block CB₂,    -   a third level of partitioning depth NP3 which contains:        -   coding blocks, such as the blocks CB₁₃ to CB₁₆ obtained on            completion of the partitioning of the block CB₁₀,        -   transform blocks, such as the blocks TB₅ to TB₈ obtained on            completion of the partitioning of the block CB₇, the blocks            TB₉ to TB₁₂ obtained on completion of the partitioning of            the block TB₂, the blocks TB₁₂ to TB₁₆ obtained on            completion of the partitioning of the block CB₁₂.

In an HEVC compatible coder, for a considered block CTB_(i), severaldifferent partitionings of said block are put in competition at thecoder, that is to say different respective combinations of partitioningiterations are put in competition, with the aim of selecting the bestpartitioning, that is to say the partitioning that optimizes the codingof the considered block CTB_(i) according to a predetermined codingperformance criterion, for example the rate/distortion cost or else anefficiency/complexity compromise, which are criteria well known to theperson skilled in the art.

Once the optimal partitioning of a considered block CTB_(i) has beencarried out, a digital information sequence, such as for example astring of bits, representative of this optimal partitioning istransmitted in a stream intended to be read by a video decoder.

Such a stream also comprises:

-   -   residual data which are the coefficients of the quantized        residual block and optionally, when coding in Inter mode,        residual data of the motion vectors,    -   coding parameters which are representative of the mode of coding        used, in particular:        -   the mode of prediction (intra prediction, inter prediction,            default prediction carrying out a prediction for which no            information is transmitted to the decoder, i.e. “skipping”),        -   information specifying the type of prediction (orientation,            reference image component, etc.);        -   the type of transform, for example 4×4 DCT, 8×8 DCT, etc.;        -   the motion information if necessary;        -   etc.

More particularly in 3D HEVC technology, it is proposed to code a firstimage component with respect to at least one second already coded andthen decoded image component, said first and second image componentsbeing representative of one and the same scene.

The aforementioned first and second image components are for example atexture component and its associated depth component, respectively, asimplemented in the new video coding format, called MVD (for “MultiviewVideo+Depth”), which is the subject of current developments.

Alternatively, the aforementioned first and second image componentscould be a depth component and its associated texture component,respectively.

In accordance with 3D HEVC technology, the first and second componentsare each partitioned into a plurality of blocks which are thereafterpartitioned as explained hereinabove. Such partitioning operations turnout to be very expensive in terms of calculations at the coder sincethey must be performed in their entirety firstly on the second componentand then on the first.

SUBJECT AND SUMMARY OF THE INVENTION

One of the aims of the invention is to remedy drawbacks of theaforementioned prior art.

For this purpose, a subject of the present invention relates to a methodfor coding at least one current block of a first image component withrespect to a reference block of at least one second image component, thefirst and second image components being representative of one and thesame scene, the reference block having previously been subjected tocoding by partitioning, and then to decoding, the partitioning of thereference block being performed a plurality of times until a determinedlevel of partitioning depth is obtained.

Such a coding method is characterized in that it comprises the stepsconsisting, as a function of the type of the first and second imagecomponents, in:

-   -   either partitioning the current block a plurality of times until        a level of partitioning depth dependent on the level of        partitioning of the reference block is obtained,    -   or partitioning the current block on the basis of a level of        partitioning depth having been previously initialized by a level        of partitioning depth which depends on the level of partitioning        depth for the reference block.

Such a provision thus makes it possible, according to the nature of thefirst and second image components:

-   -   either to halt the successive operations of partitioning of the        current block at a level of partitioning depth which is        considered to be sufficient since it complies with a        predetermined coding performance criterion, such as for example        the rate/distortion cost,    -   or to directly partition the current block on the basis of an        already established level of partitioning depth which is close        to that of the level of partitioning depth for the reference        block.

This results in a non-negligible reduction in the complexity ofcalculations at the coder which is rendered possible by virtue of thefact that the first and second image components represent one and thesame scene.

It should be noted that the first and second image components are notimage pieces but represent two different views of a complete imagerepresenting one and the same scene.

According to one particular embodiment, the step of partitioning thecurrent block, a plurality of times, is implemented when the first imagecomponent is a depth image and the second image component is a textureimage associated with the depth image, the current block and thereference block having identical positions in the first image componentand in the second image component, respectively.

Such a provision allows a reduction in complexity in terms ofcalculations at the coder when a depth image is coded with respect to atexture image.

According to another particular embodiment, the step of partitioning thecurrent block on the basis of a previously initialized level ofpartitioning depth is implemented when the first image component is atexture image and the second image component is a depth image associatedwith the texture image, the current block and the reference block havingidentical positions in the first image component and in the second imagecomponent, respectively.

Such a provision allows a reduction in complexity in terms ofcalculations at the coder when a texture image is coded with respect toa depth image.

Other types of first and second image components can of course beenvisaged.

Thus, the first and second image components can respectively be:

-   -   two views of one and the same multi-view image, said two views        representing the same scene either at the same instant or at a        different instant, or else    -   a luma component and a chroma component, or else    -   two different layers during scalable video coding, the current        block and the reference block having identical positions in the        first image component and in the second image component,        respectively.

It is also possible to envisage the coding of a first image componentwith respect to a second image component and to a third image component.In this case for example:

-   -   the first image component can be a Y component,    -   the second image component can be a U component,    -   the third image component can be a V component.

The invention also relates to a device for coding at least one currentblock of a first image component with respect to a reference block of atleast one second image component, the first and second image componentsbeing representative of one and the same scene, the reference blockhaving previously been subjected to coding by partitioning, and then todecoding, the partitioning of the reference block being performed aplurality of times until a determined level of partitioning depth isobtained.

Such a coding device is noteworthy in that it comprises:

-   -   first means for partitioning the current block which are adapted        for partitioning the latter a plurality of times until a level        of partitioning depth dependent on the level of partitioning of        the reference block is obtained,    -   second means for partitioning the current block which are        adapted for partitioning the latter on the basis of a level of        partitioning depth for the current block having been previously        initialized by a level of partitioning depth dependent on the        level of partitioning depth for the reference block, the first        and second partitioning means being activated selectively as a        function of the type of the first and second image components.

According to one particular embodiment, the first partitioning means areactivated when the first image component is a depth image and the secondimage component is a texture image associated with the depth image, thecurrent block and the reference block having identical positions in thefirst image component and in the second image component, respectively.

According to another particular embodiment, the second partitioningmeans are activated when the first image component is a texture imageand the second image component is a depth image associated with thetexture image, the current block and the reference block havingidentical positions in the first image component and in the second imagecomponent, respectively.

The invention further relates to a computer program comprisinginstructions for implementing the coding method according to theinvention when it is executed on a computer.

This program can use any programming language and be in the form ofsource code, object code or of code intermediate between source code andobject code, such as in a partially compiled form, or in any otherdesirable form.

The invention is also aimed at a recording medium which can be read by acomputer on which is recorded a computer program, this programcomprising instructions suitable for the implementation of the codingmethod according to the invention, such as described hereinabove.

The information medium can be any entity or device capable of storingthe program. For example, the medium can comprise a storage means, suchas a ROM, for example a CD ROM or a microelectronic circuit ROM, or elsea magnetic recording means, for example a USB key or a hard disk.

Moreover, the information medium can be a transmissible medium such asan electrical or optical signal, which can be conveyed via an electricalor optical cable, by radio or by other means. The program according tothe invention can in particular be uploaded to a network of Internettype.

Alternatively, the information medium can be an integrated circuit intowhich the program is incorporated, the circuit being adapted to executeor to be used in the execution of the method in question.

The aforementioned coding device and corresponding computer programexhibit at least the same advantages as those conferred by the codingmethod according to the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages will become apparent on readingpreferred embodiments described with reference to the figures, in which:

FIG. 1A shows the successive operations of partitioning a block inaccordance with HEVC technology,

FIG. 1B shows an exemplary partitioning of a coding block which has beenobtained after prediction and transformation of said block, as well asthe corresponding prediction and transformation tree,

FIGS. 2A and 2B show steps of the coding method according to theinvention,

FIG. 3 shows an embodiment of a coding device according to theinvention,

FIG. 4A shows an exemplary partitioning of a reference block of analready coded and then decoded image component, as well as the treerepresentative of the partitioning performed,

FIG. 4B shows an exemplary partitioning of a current block of an imagecomponent to be coded with respect to the partitioning of the referenceblock shown in FIG. 4A, as well as the tree representative of thepartitioning performed,

FIG. 4C shows another exemplary partitioning of a current block of animage component to be coded with respect to the partitioning of thereference block shown in FIG. 4A, as well as the tree representative ofthe partitioning performed.

DETAILED DESCRIPTION OF THE CODING METHOD OF THE INVENTION

An embodiment of the invention will now be described in which the codingmethod according to the invention is used to code a sequence of imagesaccording to a binary stream close to that which is obtained by codingaccording to the 3D HEVC standard which is being drafted. In thisembodiment, the coding method according to the invention is for exampleimplemented using software or hardware by means of modifications to acoder initially compliant with the 3D HEVC standard. The coding methodaccording to the invention is represented in the form of an algorithmcomprising steps C1 to C4 a) or else C1 to C4 b) such as are shown inFIGS. 2A and 2B.

According to the embodiment of the invention, the coding methodaccording to the invention is implemented in a coding device CO shown inFIG. 3.

As explained above in the description, with reference to FIGS. 2 and 3,the coding method according to the invention consists more particularlyin coding a first image component CI₁₁, which is the current componentof a sequence S of images (CI₁₁, CI₂₁), . . . , (CI_(1W), CI_(2W)) to becoded, with respect to at least one second image component CI₂₁, whichis a reference image component of the sequence of images to be coded,that is to say which has previously been coded, and then decoded. Thefirst image component CI₁₁ is acquired in association with the secondimage component CI₂₁, the first and second image components beingrepresentative of one and the same scene.

In the case where the image component CI₂₁ which has just been coded isa texture image and the image component CI₁₁ to be coded which isassociated with the image component CI₂₁ is a depth image, in a mannerknown per se, the components CI₂₁, . . . , CI_(2W) of the sequence S arefirst coded and then decoded. Next, the components CI₁₁, . . . , CI_(1W)of the sequence S are coded one after another in their turn.

In the case of other types of components, such as the various SVC layersor the YUV components, the action involves, in a manner known per se,the coding/decoding of the component CI₂₁, and then the coding of thecomponent CI₁₁ with reference to the component CI₂₁, and so on and soforth up until the coding/decoding of the component CI_(2W), and thenthe coding of the component CI_(1W) with reference to the componentCI_(2W).

In the course of a step CI shown in FIG. 2A, the action involves, in amanner known per se, coding by partitioning of the image component CI₂₁.

In the course of a sub-step C11 shown in FIG. 2A, the image componentCI₂₁ is partitioned into a plurality of blocks CTBr₁, CTBr₂, . . . ,CTBr_(i), . . . , CTBr_(L) of size 64×64 pixels (1≦i≦L). Suchpartitioning is performed by a partitioning software module MPCI shownin FIG. 3.

It should be noted that, within the meaning of the invention, the term“block” signifies coding unit. The latter terminology is in particularused in the HEVC standard, for example in the document “B. Bross, W.-J.Han, J.-R. Ohm, G. J. Sullivan, and T. Wiegand, “High efficiency videocoding (HEVC) text specification draft 6,” document JCTVC-H1003 ofJCT-VC, San Jose Calif., USA, February 2012”.

In particular, such a coding unit groups together sets of pixels ofrectangular or square shape, also called blocks, macroblocks, or elsesets of pixels exhibiting other geometric shapes.

In the course of a sub-step C12 shown in FIG. 2A, a block CTBr_(i) ofthe image component CI₂₁ is selected.

In the course of a sub-step C13 shown in FIG. 2A, the block CTBr_(i)selected is partitioned into a plurality of coding blocks Br₁, Br₂, . .. , Br_(j), . . . , Br_(M) with 1≦j≦M.

The aforementioned partitioning is adapted for being performed aplurality of times until a determined level k (k≧0) of partitioningdepth is obtained, for which the final partitioning obtained for theselected block CTBr_(i) optimizes a coding performance criterion, forexample, in particular the rate/distortion cost.

Said partitioning is implemented by a partitioning software module MP1shown in FIG. 3.

In the course of a sub-step C14 shown in FIG. 2A, the level ofpartitioning depth for the selected block CTBr_(i) is initialized to thevalue k.

Steps C12 to C14 are repeated for the set of blocks CTBr₁, CTBr₂, . . ., CTBr_(L).

An exemplary partitioning of the block CTBr_(i) is shown in FIG. 4A.

In the example shown, the partitioning performed complies with a tree of“quadtree” type, such as described above in the description and in whichthe level of partitioning depth k is initialized to 3.

Other types of tree can of course be envisaged.

With reference to FIG. 4A, for a given block CTBr_(i), this block isconsidered to constitute the root of a coding tree ACr in which:

-   -   a first level of leaves under the root corresponds to a first        level of partitioning depth for the block CTBr_(i) for which the        block CTBr_(i) has been partitioned a first time into a        plurality of coding blocks, for example 4 coding blocks Br₁,        Br₂, Br₃, Br₄,    -   a second level of leaves under the first level of leaves        corresponds to a second level of partitioning depth for the        block CTBr_(i) for which the block CTBr_(i) partitioned a first        time is partitioned a second time into a plurality of coding        blocks, for example 4 coding blocks Br₅, Br₆, Br₇, Br₈ arising        from the partitioning of the block Br₁,    -   a third level of leaves under the second level of leaves        corresponds to a third level of partitioning depth for the block        CTBr_(i) for which the block CTBr_(i) partitioned a second time        is partitioned a third time into a plurality of coding blocks,        for example 4 coding blocks Br₉, Br₁₀, Br₁₁, Br₁₂ arising from        the partitioning of the block Br₇.

In the course of a step C2 shown in FIG. 2A, the action involves theproduction of L sequences of bits Sr_(i), Sr₂, . . . , Sr_(i), . . . ,Sr_(L) which are representative of the partitionings performed on theblocks CTBr₁, CTBr₂, . . . , CTBr_(i), . . . , CTBr_(L), respectively.The action also involves the production of a decoded version of theblocks CTBr₁, CTBr₂, . . . , CTBr_(i), . . . , CTBr_(L), which aredenoted CTBDr₁, CTBDr₂, . . . , CTBDr_(i), . . . , CTBDr_(L) in FIGS. 2and 3. Such decoded blocks are intended to be reused by the coder CO tocode a subsequent image component, such as in particular the componentCI₁₁.

Such a step of producing binary sequences is implemented by an entropycoder CE shown in FIG. 3.

The aforementioned decoding step is for its part implemented by adecoding module MD also shown in FIG. 3.

With reference to FIG. 2B, in accordance with the invention, as afunction of the type of the first and second image components CI₁₁ andCI₂₁, two alternatives C3 a) and C3 b) for coding by partitioning areenvisaged.

In the case for example where the image component CI₂₁ which has justbeen coded is a texture image and the image component CI₁₁ to be codedwhich is associated with the image component CI₂₁ is a depth image, thecoding alternative C3 a) below is selected.

In the course of a sub-step C31 a) shown in FIG. 2B, the image componentCI₁₁ is partitioned into a plurality of blocks CTB₁, CTB₂, . . . ,CTB_(u), . . . , CTB_(S) of size 64×64 pixels (1≦u≦S). Such apartitioning step is implemented by the partitioning module MPCI shownin FIG. 3.

In the course of a sub-step C32 a) shown in FIG. 2B, a block CTB_(u) ofthe image component CI₁₁ is selected as current block to be coded withrespect to at least one coded and then decoded reference block chosenfrom among the blocks CTBr₁, CTBr₂, . . . , CTBr_(i), . . . , CTBr_(L)of the previously coded and then decoded image component CI₂₁. In thedescription that follows, the reference block chosen is for exampledeemed to be the block CTBr_(i), the current block CTB_(u) and thereference block CTBr_(i) having identical positions in the first imagecomponent CI₁₁ and in the second image component CI₂₁, respectively.

In the course of a sub-step C33 a) shown in FIG. 2B, the selected blockCTB_(u) is partitioned into a plurality of coding blocks B₁, B₂, . . . ,B_(f), . . . , B_(G) with 1≦f≦G.

In accordance with the invention, the partitioning of the block CTB_(u)is performed a plurality of times until a level k′ (k′≧0) which dependson the level of partitioning depth k for the reference block CTBr_(i) isobtained.

Such a dependency relation is expressed by the equation k′=a*k+b, wherea and b are relative integers which are predetermined at the coder.

Such a dependency relation thus allows the coding to avoidrepartitioning the block CTB_(u) an overly large number of times, thisbeing expensive in terms of complexity, while succeeding in obtainingpartitioning for the block CTB_(u) which remains in accordance with acoding performance criterion, such as for example the rate/distortioncost.

Said partitioning is implemented by said first partitioning softwaremodule MP1 shown in FIG. 3.

Steps C32 a) to C33 a) are repeated for the whole set of blocks CTB₁,CTB₂, . . . , CTB_(S).

An exemplary partitioning of the block CTB_(u) is shown in FIG. 4B.

In the example shown, the partitioning of the current block CTB_(u) isperformed at a depth level k′=2, which is lower than the level ofpartitioning depth k for the reference block CTBr_(i). For this purpose,in the relation k′=a*k+b, a and b are fixed beforehand at the coder inthe following manner: a=1 and b=−1.

Of course, according to the coding context envisaged, such as forexample the content of the image components CI₁₁ and CI₂₁, the level ofpartitioning depth k′ may be lower than or higher than the level ofpartitioning depth k by more than one level of partitioning depth. Thedependency relation k′=a*k+b is particularly advantageous in this sensesince it permits a certain flexibility of adaptation for the level ofpartitioning depth k′ according to the situations encountered duringcoding.

With reference to FIG. 4B, for a given current block CTB_(u), this blockis considered to constitute the root of a coding tree AC in which:

-   -   a first level of leaves under the root corresponds to a first        level of partitioning depth for the block CTB_(u) for which the        block CTB_(u) has been partitioned a first time into a plurality        of coding blocks, for example 4 coding blocks B₁, B₂, B₃, B₄,    -   a second level of leaves under the first level of leaves        corresponds to a second level of partitioning depth for the        block CTB_(u) for which the block CTB_(u) partitioned a first        time is partitioned a second time into a plurality of coding        blocks, for example 4 coding blocks B₅, B₆, B₇, B₈ arising from        the partitioning of the block B₁.

In the example shown, the block B₇ has therefore not been repartitionedinto 4 blocks like the block Br₇ of the reference block CTBr_(i).

In the course of a step C4 a) shown in FIG. 2B, the action involves theproduction of S sequences of bits S₁, S₂, . . . , S_(u), . . . , S_(S)which are representative of the partitionings performed on the blocksCTB₁, CTB₂, . . . , CTB_(u), . . . , CTB_(S), respectively. The actionalso involves the production of a decoded version of the blocks CTB₁,CTB₂, . . . , CTB_(u), . . . , CTB_(S), which are denoted CTBD₁, CTBD₂,. . . , CTBD_(u), . . . , CTBD_(S) in FIG. 2B. Such decoded blocks areintended to be reused by the coder CO to code a subsequent imagecomponent.

Such a step of producing binary sequences is implemented by the entropycoder CE shown in FIG. 3.

The aforementioned decoding step is for its part implemented by thedecoding module MD also shown in FIG. 3.

The second alternative C3 b) for coding by partitioning which isenvisaged in accordance with the invention will now be described withreference to FIG. 2B.

Such an alternative is envisaged in the case for example where the imagecomponent CI₂₁ which has just been coded is a depth image and the imagecomponent CI₁₁ to be coded which is associated with the image componentCI₂₁ is a texture image.

In the course of a sub-step C31 b) shown in FIG. 2B, the image componentCI₁₁ is partitioned into a plurality of blocks CTB′₁, CTB′₂, . . . ,CTB′_(u), . . . , CTB′_(S) of size 64×64 pixels (1≦u≦S). Such apartitioning step is implemented by the partitioning module MPCI shownin FIG. 3.

In the course of a sub-step C32 b) shown in FIG. 2B, a block CTB′_(u) ofthe image component CI₁₁ is selected as current block to be coded withrespect to at least one coded and then decoded reference block chosenfrom among the blocks CTBr₁, CTBr₂, . . . , CTBr_(i), . . . , CTBr_(L)of the previously coded image component CI₂₁. In the description thatfollows, the reference block chosen is for example deemed to be theblock CTBr_(i), the current block CTB′_(u) and the reference blockCTBr_(i) having identical positions in the first image component CI₁₁and in the second image component CI₂₁, respectively.

In the course of a sub-step C33 b) shown in FIG. 2B, the action involvesthe initialization of the level of partitioning depth for the blockCTB′_(u), on the basis of the level of partitioning depth k′=a*k+b.

For this purpose, in the course of a step C34 b) shown in FIG. 2B, theblock CTB′_(u) is partitioned directly into a plurality of coding blocksB′₁, B′₂, . . . , B′_(g), . . . , B′_(H) with 1≦g≦H, on the basis of thepreviously initialized level of partitioning depth k′. Intermediatesteps of partitioning of the block CTB′_(u) are thus avoided, therebynoticeably reducing the complexity in terms of calculations duringcoding.

Subsequent to the step C34 b) of partitioning of the block CTB′_(u), thelatter can be partitioned again a plurality of times until apredetermined level of partitioning depth is obtained at the coder.

Said partitioning is implemented by a second partitioning softwaremodule MP2 shown in FIG. 3.

Steps C32 b) to C34 b) are repeated for the whole set of blocks CTB′₁,CTB′₂, . . . , CTB′_(S).

An exemplary partitioning of the block CTB′_(u) is shown in FIG. 4C.

In this example, the reference block CTBr_(i) used to code the currentblock CTB′_(u) is deemed to be that which has been partitioned as shownin FIG. 4A.

In the example shown in FIG. 4C, the partitioning of the current blockCTB′_(u) is performed on the basis of a depth level k′=4, which ishigher than the level of partitioning depth k for the reference blockCTBr_(i). For this purpose, in the relation k′=a*k+b, a and b are fixedbeforehand at the coder in the following manner: a=1 and b=1. As hasbeen explained in the description above, other choices of level ofpartitioning depth k′ are possible according to the video context. Inparticular, the level of partitioning depth k′ can be lower than orhigher than the level of partitioning depth k by more than one level ofpartitioning depth.

With reference to FIG. 4C, for a given current block CTB′_(u), thisblock is considered to constitute the root of a coding tree AC′ inwhich:

-   -   a first level of leaves under the root corresponds to a first        level of partitioning depth for the block CTB′_(u) for which the        block CTB′_(u) has been partitioned a first time into a        plurality of coding blocks, for example 4 coding blocks B′₁,        B′₂, B′₃, B′₄,    -   a second level of leaves under the first level of leaves        corresponds to a second level of partitioning depth for the        block CTB′_(u) for which the block CTB′_(u) partitioned a first        time is partitioned a second time into a plurality of coding        blocks, for example 4 coding blocks B′₅, B′₆, B′₇, B′₈ arising        from the partitioning of the block B′₁,    -   a third level of leaves under the second level of leaves        corresponds to a third level of partitioning depth for the block        CTB′_(u) for which the block CTB′_(u) partitioned a second time        is partitioned a third time into a plurality of coding blocks,        for example 4 coding blocks B′₉, B′₁₀, B′₁₁, B′₁₂ arising from        the partitioning of the block B′₇,    -   a fourth level of leaves under the third level of leaves        corresponds to a fourth level of partitioning depth for the        block CTB′_(u) for which the block CTB′_(u) partitioned a third        time is partitioned a fourth time into a plurality of coding        blocks, for example 4 coding blocks B′₁₃, B′₁₄, B′₁₅, B′₁₆        arising from the partitioning of the block B′₁₁.

In the example shown, the block B′₁₁ has therefore been partitioned into4 blocks in contrast with the block Br_(1i) of the reference blockCTBr_(i).

In the course of a step C4 b) shown in FIG. 2B, the action involves theproduction of S sequences of bits S′₁, S′_(u), . . . , S′_(S) which arerepresentative of the partitionings performed on the blocks CTB′₁,CTB′₂, . . . , CTB′_(u), . . . , CTB′_(S), respectively. The action alsoinvolves the production of a decoded version of the blocks CTB′₁, CTB′₂,. . . , CTB′_(u), . . . , CTB′_(S), which are denoted CTBD′₁, CTBD′₂, .. . , CTBD′_(u), . . . , CTBD′_(S) in FIG. 2B. Such decoded blocks areintended to be reused by the coder CO to code a subsequent imagecomponent.

Such a step of producing binary sequences is implemented by the entropycoder CE shown in FIG. 3.

The aforementioned decoding step is for its part implemented by thedecoding module MD also shown in FIG. 3.

It goes without saying that the embodiments which have been describedhereinabove have been given purely by way of wholly non-limitingindication, and that numerous modifications can be easily introduced bythe person skilled in the art without, however, departing from the scopeof the invention.

The invention claimed is:
 1. A method comprising: coding at least onecurrent block of a first image component with respect to a referenceblock of at least one second image component, said first and secondimage components being representative of one and the same scene, saidreference block having previously been subjected to coding bypartitioning, and then to decoding, said partitioning of the referenceblock being performed a plurality of times until a determined level (k)(k≧0) of partitioning depth is obtained, said method comprising, beforesaid coding, as a function of the type of the first and second imagecomponents: either partitioning by a coding device the current block tobe coded a plurality of times until a level of partitioning depth (k′)dependent on the level of partitioning (k) of the reference block isobtained, by using a dependency relation that is predetermined, orpartitioning by the coding device the current block to be coded on thebasis of a level of partitioning depth having been previouslyinitialized by a level of partitioning depth (k′) which depends on thelevel of partitioning depth (k) for the reference block by using adependency relation that is predetermined, wherein partitioning thecurrent block, a plurality of times, is implemented when the first imagecomponent is a depth image and the second image component is a textureimage associated with said depth image, the current block and thereference block having identical positions in the first image componentand in the second image component, respectively.
 2. The coding method asclaimed in claim 1, in which partitioning the current block on the basisof a previously initialized level of partitioning depth is implementedwhen the first image component is a texture image and the second imagecomponent is a depth image associated with said texture image, thecurrent block and the reference block having identical positions in thefirst image component and in the second image component, respectively.3. A device for coding at least one current block of a first imagecomponent with respect to a reference block of at least one second imagecomponent, said first and second image components being representativeof one and the same scene, said reference block having previously beensubjected to coding by partitioning, and then to decoding, saidpartitioning of the reference block being performed a plurality of timesuntil a determined level (k) (k≧0) of partitioning depth is obtained,said device comprising: first means for partitioning the current blockto be coded which are adapted for partitioning the latter a plurality oftimes until a level of partitioning depth (k′) dependent on the level ofpartitioning (k) of the reference block is obtained, by using adependency relation that is predetermined, second means for partitioningthe current block to be coded which are adapted for partitioning thelatter on the basis of a level of partitioning depth for the currentblock having been previously initialized by a level of partitioningdepth (k′) dependent on the level of partitioning depth (k) of thereference block, by using a dependency relation that is predetermined,said first and second partitioning means being activated before thecoding of said at least one current block, selectively as a function ofthe type of the first and second image components, and wherein saidfirst partitioning means are activated when the first image component isa depth image and the second image component is a texture imageassociated with said depth image, the current block and the referenceblock having identical positions in the first image component and in thesecond image component, respectively.
 4. The coding device as claimed inclaim 3, in which said second partitioning means are activated when thefirst image component is a texture image and the second image componentis a depth image associated with said texture image, the current blockand the reference block having identical positions in the first imagecomponent and in the second image component, respectively.
 5. Anon-transitory recording medium which can be read by a computer on whichis recorded a computer program comprising instructions for execution ofa method of coding method for coding at least one current block of afirst image component, when this program is executed by a processor,wherein the method comprises: coding the at least one current block ofthe first image component with respect to a reference block of at leastone second image component, said first and second image components beingrepresentative of one and the same scene, said reference block havingpreviously been subjected to coding by partitioning, and then todecoding, said partitioning of the reference block being performed aplurality of times until a determined level (k) (k≧0) of partitioningdepth is obtained, said method comprising, before said coding, as afunction of the type of the first and second image components: eitherpartitioning by a coding device the current block to be coded aplurality of times until a level of partitioning depth (k′) dependent onthe level of partitioning (k) of the reference block is obtained, byusing a dependency relation that is predetermined, or partitioning bythe coding device the current block to be coded on the basis of a levelof partitioning depth having been previously initialized by a level ofpartitioning depth (k′) which depends on the level of partitioning depth(k) for the reference block by using a dependency relation that ispredetermined, wherein partitioning the current block, a plurality oftimes, is implemented when the first image component is a depth imageand the second image component is a texture image associated with saiddepth image, the current block and the reference block having identicalpositions in the first image component and in the second imagecomponent, respectively.
 6. An apparatus comprising: a non-transitorycomputer-readable storage medium; and a data sequence stored on thestorage medium and representative of a partitioning of at least onecurrent block of a first image component which has been coded withrespect to a reference block of at least one second image component,said first and second image components being representative of one andthe same scene, said reference block having previously been subjected tocoding by partitioning, and then to decoding, said partitioning of thereference block being performed a plurality of times until a determinedlevel (k) (k≧0) of depth of partitioning is obtained, wherein said datasequence is representative, before coding of said at least one currentblock, as a function of the type of the first and second imagecomponents: either of the partitioning of the current block to be codeda plurality of times until a level of depth of partitioning (k′)dependent on the level of partitioning (k) of the reference block isobtained, by using a dependency relation that is predetermined, or ofthe partitioning of the current block to be coded on the basis of alevel of depth of partitioning having been previously initialized by alevel of depth of partitioning (k′) which depends on the level of depthof partitioning (k) for the reference block by using a dependencyrelation that is predetermined, wherein in the case where said datasequence is representative of partitioning the current block a pluralityof times, the first image component is a depth image and the secondimage component is a texture image associated with said depth image, thecurrent block and the reference block having identical positions in thefirst image component and in the second image component, respectively.